In a DNA molecule, the backbone is composed of what molecules

Look at the diagrams at this Univ of Arizona biochem site
http://www.blc.arizona.edu/molecular_gra…

A phosphate group composed of 1 phosphate, 2 neutral oxygen atoms, and 2 negatively charged oxygen atoms. one of the negatively charge oxygen atoms binds to a pentose sugar and becomes neutral which then binds to another negatively charged oxygen from a phosphate group.

Simply, phosphate groups and pentose sugars. In DNA, the pentose sugar is a deoxyribose sugar because it lacks one oxygen which is teh main difference between the pentose sugar in DNA and RNA

Deoxyribose sugars and phosphate groups

phosphate and sugar (deoxyribose)